Multi-cell batteries that are constructed in a broad range of electrochemical systems are typically packaged in cylindrical or prismatic housings. Individual cells are connected in series by conductive links to make the multi-cell batteries. Such construction approaches provide for good sealing of the individual cell compartments and for reliable operation. However, such constructions allocate a large fraction of the multi-cell battery's weight and volume to the packaging and, thus, do not make full use of the potential energy storage capacity of the active components of the cell. For improving battery energy storage capacity on a weight and volume basis, packaging approaches are sought that reduce packaging weight and volume and that provide stable battery performance and low internal resistance.
These objectives have led to the pursuit of a bipolar construction in which an electrically conductive bipolar layer serves as the electrical interconnection between adjacent cells as well as a partition between the cells. In order for the bipolar construction to be successfully utilized, the bipolar layer must be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good electrical contact to the electrodes and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell. These requirements are more difficult to achieve in rechargeable batteries due to the charging potential that can accelerate corrosion of the bipolar layer and in alkaline batteries due to the creep nature of the electrolyte. Achieving the proper combination of these characteristics has proved very difficult.
For maintenance-free operation it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that are structurally poor for containment of the gases present or generated during cell operation. In a sealed cell construction, gases are generated during charging that need to be chemically recombined within the cell for stable operation. To minimize weight of the structures used to provide the gas pressure containment, the battery should operate at relatively low pressure. The pressure-containment requirement creates additional challenges on designing a stable bipolar configuration.
Despite a number of patents and considerable effort at making a bipolar construction for the lead-acid and nickel-cadmium systems such batteries are not commercially available, (U.S. Pat. No. 4,098,967). Construction of a flat metal hydride battery is even more difficult because many of the metal hydride alloys used to make metal hydride batteries operate at elevated hydrogen pressures.
The bipolar construction has been successfully employed in the flat plate construction of the Leclanche MnO.sub.2 --Zn system as a primary battery, U.S. Pat. No. 4,098,965. Since a primary battery is non-rechargeable, the materials-stability problem is less severe and the aqueous chloride electrolyte may be contained without unreasonable difficulty.
Another problem of prior art electrochemical cells relates to the material problems encountered with metal hydride electrodes. Electrochemically reversible metal hydride electrodes operate by the absorption of hydrogen in the lattice of the metal hydride alloy during electrochemical charging of the cell. A number of alloy formulations have been identified of the so-called AB.sub.5 and AB.sub.2 structure that can function in this manner, for example, as disclosed in U.S. Pat. Nos. 4,487,817 and 4,728,586. To insure reasonable rates of reaction and transport of hydrogen, such electrodes may be prepared from alloy powders typically having an average particle size of about 50 microns. Fabricating an electrode structure from the alloy powders may be accomplished by sintering the metal powders or by using polymeric binders. However, conventional techniques do not yield electrodes that make good and stable contact to the cell face of the conductive outer layer in a bipolar construction. Metal hydride alloys can fragment during repeated cycling as the alloy undergoes expansion and contraction each time the hydrogen enters and leaves the lattice. It is also recognized that oxygen and or the electrolyte can react with the hydride alloy and cause deterioration of the hydrogen storage capacity of the hydride alloy.
The present invention provides a method for achieving the packaging benefits of bipolar construction for rechargeable multi-cell batteries and of overcoming the materials and construction problems of previous approaches. Although the materials of construction for each type of cell are specific to each battery chemistry, the generic bipolar construction disclosed herein may be used for many types of electrochemical cells. In particular, the descriptions and approaches that follow relate specifically to the rechargeable nickel-metal hydride chemistry but may generally be adaptable to other chemistries.